Nitrogen isotope composition of ammonium in PM in the Xiamen, China: impact of non-agricultural ammonia.

Since NH is a significant precursor to ammonium in PM and contributes significantly to atmospheric nitrogen deposition but largely remains unregulated in China, the insight into the source of NH emissions by the isotopic investigation is important in controlling NH emissions. In this study, atmospheric concentrations of NH and water-soluble ion composition in PM as well as nitrogen isotope ratios in NH (δN-NH) in Xiamen, China, were measured. Results showed that average NH concentration for the five sites in Xiamen was 7.9 μg m with distinct higher values in the warm season and lower values in the cold season, and PM concentration for the two sites (urban and suburban) was 59.2 μg m with lowest values in summer. In the PM, NH concentrations were much lower than NH and showed a stronger positive correlation with NO than that with SO suggesting the formation of NHNO and equilibrium between NH and NH. Although the concentrations of NH at the urban site were significantly higher than those at the suburban site, no significant spatial difference in NH and δN-NH was obtained. The distinct heavier δN-NH values in summer than in other seasons correlated well with the equilibrium isotopic effects between NH and NH which depend on temperature. The initial δN-NH values were in the range of waste treatment (- 25.42‰) and fossil fuel combustion (- 2.5‰) after accounting for the isotope fractionation. The stable isotope mixing model showed that fossil fuel-related NH emissions (fossil fuel combustion and NH slip) contributed more than 70% to aerosol NH. This finding suggested that the reduction of NH emissions from urban transportation and coal combustion should be a priority in the abatement of PM pollution in Xiamen.